Bubbler for use in vapor generation systems

ABSTRACT

A bubbler for use in vapor generation systems that minimizes splashing and the formation of aerosol droplets of liquid, which are carried out of the bubbler in the vapor stream and result in erratic mass transfer of the process chemical. A closed stainless steel vessel contains a carrier gas distribution plenum that distributes the carrier gas to a plurality of small diameter generator tubes, which are submerged into the process chemical. The length, inside diameter and number of the generator tubes are designed to inject a high velocity, small diameter stream of carrier gas into the liquid such that a long small diameter cylinder of carrier gas is created in the liquid. The surface tension of the liquid-gas interface causes the cylinder of gas to be pinched off at intervals along the length of the cylinder to produce a plurality of small bubbles the diameter of which is largely independent of the carrier gas flow rate. By preventing the formation of large diameter bubbles at high carrier gas flow rates splashing and the formation of aerosol droplets of liquid are effectively eliminated.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] This invention relates to bubblers for supplying a vapor to achemical process by introducing a carrier gas into a liquid processchemical.

[0003] 2. Description of Related Art

[0004] U.S. Pat. No. 5,078,922 of Collins et al. shows a “Liquid SourceBubbler.”

[0005] U.S. Pat. No. 5,921,428 of Rodgers shows a “Self-MeteringReservoir”.

SUMMARY OF THE INVENTION

[0006] An object of the invention is to provide a bubbler thatsuppresses splashing and the generation of aerosol droplets at highcarrier gas flow rates, which exit the bubbler in the outlet flowmixture of carrier gas and chemical vapor, thus creating erraticvariations in chemical mass transfer.

[0007] Another object of the invention is to provide the high flow rate,anti-aerosol properties with a bubbler having a small internal volume.

[0008] A further object of the invention is to provide a high flow ratesmall volume bubbler, whose outlet concentration of chemical vapor tocarrier gas is independent of the carrier gas flow rate.

[0009] Another object of the invention is to provide a high flow ratesmall volume bubbler, whose outlet concentration of chemical vapor tocarrier gas is largely independent of the liquid level in the bubbler.

[0010] In accordance with the above objects, the invention provides anapparatus and method for generating a saturated mixture of a carrier gasand a chemical vapor devoid of chemical liquid droplets. The bubblerconsists of a closed stainless steel bubbler container having a carriergas inlet tube, a carrier gas/vapor outlet, a chemical liquid fill inletand a chemical liquid drain outlet. The carrier gas inlet tube passesthrough the top of the bubbler container and into an enclosed plenumthat distributes the carrier gas to a plurality of small generatortubes. The generator tubes extend from the bottom of the plenum downinto the chemical liquid in the bubbler container. The dimensions of thegenerator tubes are chosen such that at the maximum carrier gas flowrate the carrier gas stream exiting the generator tube into the liquidis a high velocity fully developed laminar flow comprising a cylindricalstream. Under these conditions the exiting cylindrical stream of carriergas maintains a small diameter cylindrical shape in the chemical liquidfor a substantial distance from the outlet end of the generator tube. Asthe stream stretches farther away from the outlet end of the generatortube, the surface tension at the carrier gas chemical liquid interfaceacts to pinch off the cylindrical stream of carrier gas into a series ofsmall bubbles whose diameter is primarily a function of the diameter ofthe cylindrical stream of carrier gas and the surface tension. Thebubble diameter is almost independent of flow rate. The series of smallbubbles rises up through the chemical liquid and quickly becomes fullysaturated with chemical vapor due to their large surface-area-to-volumeratio. A further benefit of maintaining small bubble size is that therate of bubble ascent is limited, thus increasing contact time with thechemical liquid while minimizing splashing when the bubble breaks thesurface of the chemical liquid. The carrier gas vapor outlet tube passesthrough the top of the bubbler container and is located behind theplenum such that the plenum acts as a baffle to shield the carrier gasvapor outlet from the surface of the chemical liquid as a further meansof preventing any liquid from entering the outlet stream.

[0011] Chemical liquid level measurement means measure the chemicalliquid level inside the bubbler container to provide for chemical liquidlevel alarm conditions and for automatic filling. A piezoceramictransducer is bonded to the outside surface of the bottom of the bubblercontainer in an area aside from the location of the generator tubes. Anelectrical signal is applied to the piezoceramic transducer thatgenerates an elastic wave that propagates through the bottom of thestainless steel bubbler container and into the chemical liquid. Theacoustical wave propagates through the chemical liquid and is almosttotally reflected at the surface of the chemical liquid due to themismatch in acoustical impedance between a liquid and a gas. Thereflected acoustical wave propagates back through the liquid and thebottom of the bubbler container and is received by the piezoceramictransducer, thereby producing an electrical signal, which is detectedand processed to determine the time delay between the transmitted andreceived signals. The height of the liquid above the piezoceramictransducer is calculated as a function of the measured time delay andthe known speed of sound in the liquid. Because the speed of sound in aliquid is almost independent of the chemical composition of the liquid,a generic speed of sound of 1,300 meters per second can be used andstill maintain a liquid level measurement accuracy of ±10%.

[0012] The column of chemical liquid above the piezoceramic transduceris partially isolated from the bulk of the chemical liquid volume by astainless steel baffle attached to the inside wall of the bubblercontainer. The baffle keeps the chemical liquid surface above thepiezoceramic transducer relatively smooth, further enhancing theaccuracy of the time delay measurement. Small gaps at the top and bottomof the baffle connect the volume enclosed by the baffle with the rest ofthe volume of the bubbler container, thus allowing the height H′ of thechemical liquid level in the volume enclosed by the baffle to remain inequilibrium with the height H of the chemical liquid level in the mainvolume of the bubbler container.

[0013] Temperature control means allow bubbler operation above ambienttemperature to increase the outlet concentration of chemical vapor inthe carrier gas. These include a molded silicon-rubber insulating jacketthat encapsulates the bubbler container and inlet and outlet fittings,heating elements bonded to the exterior surfaces of the bubblercontainer, a temperature measurement means and a temperature feedbackcontrol means.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014]FIG. 1 is a top view of a bubbler in accordance with the presentinvention.

[0015]FIG. 2 is a sectional view taken along the section line A-A inFIG. 1.

[0016]FIG. 3 is a detail view C in FIG. 2 illustrating the bubbleformation process in accordance with the present invention.

[0017]FIG. 4 is a detail view D in FIG. 2 illustrating the gap at thebottom of the baffle and the piezoelement.

[0018]FIG. 5 is a sectional view taken along the section line B-B inFIG. 1.

[0019]FIG. 6 is a detail view E in FIG. 5 illustrating the temperaturesensor inside a temperature sensor well.

[0020]FIG. 7 is a schematic diagram showing the control systems andfluid flow systems of the present invention.

[0021]FIG. 8 is a flow chart of a program controlling the fluid levelcontrol system of this invention.

DETAILED DESCRIPTION OF THE INVENTION AND ITS PREFERRED EMBODIMENTS

[0022] For ease of discussion, the following description of theinvention and its preferred embodiments of the invention will referencethe accompanying drawings, it being understood that the describedpreferred embodiments are not intended to limit the scope of theinvention as defined by the appended claims.

[0023]FIG. 1, shows a top view of the entire bubbler 31. FIG. 2 is asectional view taken along line A-A of FIG. 1 which illustrates thebasic elements of the invention. The bubbler 31 includes a bubblercontainer 30 consisting of a side wall 32, a bottom 33 and a top 34. Thebubbler container 30 is enclosed in an insulated heating jacket 43,(surrounding the side wall 32, bottom 33 and top 34) such as the typemade by Watlow comprised of sidewalls 44, base 35 and top 36. Atemperature sensor 37 inside a temperature sensor well 38 senses thetemperature of the chemical liquid 14 as shown in FIG. 5 and FIG. 6. Thetemperature sensor 37 can, for example, be a thermistor, such as aTO501/B2-P60BB103M-CQOGA manufactured by Thermometrics, Edison N.J.

[0024] Referring to FIG. 7, the bubbler container 30 is shown with a gassource 42 connected by fitting 41 through line 40 to the gas inletfitting 39. The bubbler container 30 has a gas outlet 19 for the gasprovided by the bubbler 31. A reservoir 95 for chemical liquid is shownconnected by fitting 94, valve 93 and fluid inlet fitting 29 to thebubbler container 30. A heating jacket 52 surrounds the bubblercontainer 30. A temperature sensor 37 is connected by a cable 50 to atemperature controller 51 which is connected to provide power to theheating jacket 52 by cables 53A/53B. The temperature controller 51controls provision of power to the jacket 52 to maintain the temperatureof the chemical liquid 14 at or near a preset temperature value. Anexample of such a temperature control unit is the model 96A temperaturecontroller manufactured by Watlow. The temperature controller 51 isconnected by lines 53A and 53B to the heating jacket 52 which surroundsthe bubbler container 30.

[0025] The bubbler container 30 including the side wall 32, bottom 33and top 34, the inlet fittings 39 and the interior components of thebubbler 31 are composed of a high purity, corrosion resistivematerial(s), such as stainless steel, quartz, a fluoropolymer, or thelike. Welded stainless steel, e.g., 316L stainless steel, is aparticularly preferred material for the bubbler 31. A carrier gas entersthe bubbler 31 at a controlled mass flow rate through a gas inletfitting 39, and flows directly into an enclosed distribution plenum 10comprised of a plenum cap 11 and plenum base 12 which defines a plenumvolume.

[0026] The distribution plenum 10 supplies the carrier gas to aplurality of small diameter generator tubes 13 that extend from theplenum base 12 down into the volume of chemical liquid 14. The design ofthe generator tubes 13 simultaneously satisfies the requirements thatthe length of a generator tube 13 is approximately greater than onehundred times the internal diameter of the generator tube 13 and at themaximum rated carrier gas flow rate of the bubbler the Reynolds numberof the flow inside a generator tube 13 is typically less than 1000.

EXAMPLE

[0027] We have found that for a maximum carrier gas flow rate of 20standard liters per minute of nitrogen gas and a bubbler operatingtemperature and pressure of 60° C. and 760 torr, with 52 generator tubes13 having an ID of 0.07874 cm (0.031 inches) and a length of 8.890 cm(3.5 inches) results in a Reynolds number of nominally 700.

[0028] The carrier gas flow exits from the generator tubes 13 with afully developed pattern of laminar flow and forms a cylindrical stream15 of gas extending from the end of the generator tube 13 down into thechemical liquid. As the stream stretches farther away from the outletend of the generator tube, the surface tension at the liquid gasinterface causes the cylindrical stream 15 of gas to be pinched off atconsistent intervals to form a stream of consistently sized bubbles 16having diameters on the order of 1.6 times that of the ID of thegenerator tube 13.

[0029] Detail C of FIG. 2 shown in FIG. 3 in an enlarged view of thebubble formation process. The rate at which an ascending gas bubble 16becomes saturated with the chemical vapor increases as the ratio ofbubble surface area to volume increases. Hence smaller diameter bubbles16 saturate more quickly than larger diameter bubbles 16 . Additionally,the buoyancy of a bubble 16 is proportional to the cube of the diameterof the bubble 16. While the drag on an ascending bubble 16 isproportional to the square of the diameter of the bubble 16, smallerdiameter bubbles 16 ascend more slowly than larger diameter bubbles 16.

[0030] Thus, as the bubbles become smaller, the contact time of thebubbles 16 with the chemical liquid 14 increases. Thus the smalldiameter of the bubbles 16 minimizes splashing when a bubble 16 breaksthe surface 17 of the chemical liquid 14 with the surface 17 a height Habove the bottom 33 of the bubbler container 30.

[0031] A mixture of carrier gas saturated with chemical vapor exits fromthe bubble bubbler container 30 through outlet port 18 and gas outletfitting 19, e.g. VCR™ A plenum cap 11 is attached to the top 34 of thebubbler 31 by three welded spacers 20 spaced 120° apart (only one ofwhich is shown in FIG. 2 for convenience of illustration) to form theshielded volume 21 that supplies the gas vapor mixture to outlet port18.

[0032] The level of the chemical liquid is sensed by a sensing means,such as a transducer 25 such as an acoustical level sensor, optical orthermal level detectors. Acoustical level sensing is a preferred methodbecause it is non invasive and it has the inherent capability of sensinga continuum of chemical liquid levels rather than a few discrete levels.Referring to FIGS. 1, 2 and 4, a baffle 22 which is attached to thesidewall 32 of the bubbler 31, partially isolates a column 22′ ofchemical liquid 14 from the main volume of the bubbler 31 and keeps thesurface 23 of this column 22′ relatively smooth compared to the bubbleagitated surface 17 of the main volume of the chemical liquid 14. Smallgaps 24 at the top and bottom of the baffle 22 connect the volumeenclosed by the baffle 22 with the rest of the volume of the bubblercontainer 30, thus allowing the level H′ of the chemical liquid 14 inthe column 22′ enclosed by the baffle 22 to remain in equilibrium withthe level H of the chemical liquid 14 in the main volume contained inthe bubbler container 30. The transducer 25 is preferably apiezoelement, e.g. APC 850 manufactured by American Piezoceramics, Inc.,Mackeyville, Pa. has a diameter of 12.7 mm (0.5 inch), a thickness of1.0 mm (0.04 inches), and an acoustical resistance of 30.5×10⁶Pa·s/m.The piezoelement transducer 25 has a radial mode resonance frequency ofapproximately 160 kHz and a thickness mode resonance frequency of 2 MHzis bonded to the outside surface of the bottom 33 of the bubbler 31using a high temperature epoxy, e.g low viscosity resin and hardenerfrom Duralco 4461 which is mixed and cured in accordance with theinstructions of the manufacturer; Cotronics Corporation, Brooklyn, N.Y.

[0033] The piezoelement transducer 25 is located under the column 22′ ofchemical liquid 14 enclosed by the baffle 22. Piezoelement transducer 25serves as both an acoustical transmitter and receiver convertingelectrical vibrations into mechanical vibrations and converting receivedmechanical vibrations into electrical vibrations. A pulse of electricalenergy having a center frequency of 2 MHz is applied to the piezoelementtransducer 25 generating a compressional elastic pulse that propagatesthrough the bottom 33 of container 30 and into the chemical liquid 14.Acoustical energy is transmitted relatively efficiently between thepiezoelement transducer 25, stainless steel bottom 33 of bubblercontainer 30 of the bubbler 31 and the chemical liquid 14 due to theirrelatively well matched acoustical impedances of 3.5×10⁶, 40×10⁶ and2×10⁶ Pa·s/m respectively. The compressional acoustic wave propagatesvertically up through the chemical liquid 14 and is reflected at thechemical liquid 14 surface 23 due to the large impedance mismatch atthis liquid/gas interface; the acoustical impedance of a gas istypically in the range of from about 30Pa·s/m to about 400Pa·s/m versus2×10⁶Pa·s/m for a liquid. The reflected acoustical pulse propagates downthrough the chemical liquid 14, through the bottom 33 of the bubbler 31and therefrom into the piezoelement transducer 25, whereupon it isconverted into electrical vibrations that are detected and processed byelectronic circuitry shown in FIG. 7 that is connected to the transducer25 by a cable 26.

[0034] Referring again to FIG. 7, a digital signal processor (DSP) 60provides pulse generating signals which are connected by bus line 64 tothe data buffer 61A that is connected to digital-to-analog converter 61Bwhich sends analog pulses to tranmitter amplifier 61C that sends ananalog pulse to multiplexer 62, which send the pulse on the cable 26 toenergize the transducer 25. When the transducer 25 receives themechanical vibrations and converts them into electrical vibrations, itgenerates analog output pulses connected by cable 26 to the multiplexer62 which sends its output to analogy receiver amplifier 63A thatprovides an output to digital-to-analog converter 63B which suppliesdigital output pulses to data buffer 63C which is connected to the DSP60 for processing. The DSP 60 employs programs stored in a computerstorage device, e.g. EPROM 65 (comprising a non-volatile memory). EPROMcontains a liquid level control program of the kind shown in FIG. 8.Data can be entered into the DSP 60 with the numeric display and keypad66 or the equivalent, as will be well understood by those skilled in theart. When the DSP 60 determines, by using the computer program of FIG.8, that the level H′ is too low, then a signal is sent on line 90 to theCVD tool process control computer 91 causing it to send an output1 online 92 energizing the control valve 93 which supplies chemical liquid14 to the bubbler container 30 until the level H′ reaches the desiredlevel.

[0035] The program which performs the task of determining the level H′is shown in FIG. 8, which starts with step 70. In step 71, the programcauses the DSP 60 to generate a 2MHz signal at time t1 (with thepiezo-ceramic transistor operated in its thickness mode). Then, in step72 the program records the time t2 of detection the return pulse fromtransducer 25. In step 74, the value of H′ is calculated. In step 75,the program tests whether the value of H′ is too low. If YES, theprogram proceeds to step 76 in which the DSP 60 generates a signal online 90 to cause computer 91 to open valve 93 to add chemical liquid 14to bubbler container 30. Then the program proceeds to step 78 whichagain tests whether the value of H′ is too low. If the result of thetest in step 78 is NO, then the program proceeds to step 80 which causesthe DSP 60 to generate a signal on line 90 causing computer 91 to closethe valve 93. If the result of the test in step 78 is YES, then theprogram proceeds on line 79 to repeat the program starting with step 71.

[0036] If step 75 issues a NO answer, then the program proceeds on line77 to the end 78 of the routine. The program of FIG. 8 is repeated on arepetitive cycle under control of the clock in the DSP 60 to assure thatthe level required is maintained.

[0037] The height H′ of the chemical liquid 14 in the column 22′ iscomputed as follows:

H^(t)=C_(t) πt/2

[0038] where:

[0039] C_(t) is the speed of sound in the chemical liquid 14,

[0040] πt is the time delay between the transmitted and received pulseand

[0041] the factor of {fraction (1/2 )} accounts for the fact that theacoustical pulse traversed

[0042] the column 22′ of chemical liquid 14 two times.

[0043] Because the speed of sound in most liquids is almost constant,e.g., 1300±100 meters/second a fixed speed of sound can be assumed andstill maintain a liquid level measurement accuracy of ±10%. If a higherdegree of accuracy is required, the true speed of sound within theactual chemical liquid 14 can be used in the calculation. Systematictime delays can be measured during calibration and removed from thesubsequent calculations. The measured level of the chemical liquid 14can be used for reporting alarm conditions as well as automating thefilling of the bubbler container 30 with the chemical liquid 14.Automated filling is segmented into two commonly used methods:

[0044] i) batch fill in which the level of the chemical liquid 14 isallowed to reach some minimum value at which point a valve 93 is openedallowing chemical liquid 14 to flow into the bubbler container 30 untilthe level 23 of chemical liquid 14 reaches a value that is deemed to bethe “bubbler full” condition and the external valve 93 is closed or;

[0045] ii) chemical liquid 14 is metered into the bubbler container 30to keep the liquid level 23 at some quiescent operating level.

[0046] Referring to FIG. 5, a combination chemical liquid 14 fill anddrain tube 27 extends through the bubbler top 34 and down to a recessedarea 28 machined into the inside surface of the bottom 33 of the bubblercontainer 30. The fill/drain tube 27 is connected to the fill/drainfitting 29, e.g. a VCR™ fill/drain fitting manufactured by SwagelokCompany, 29500 Solon Road, Solon, Ohio 44139.

[0047] Lorex fabricated a bubbler 31 in accordance with the presentinvention, as described above, and tested its performance at flow ratesup to 50 standard liters per minute using nitrogen as a carrier gas andisopropyl alcohol as the chemical liquid 14. A Lorex Piezocon™acoustical gas concentration sensor was connected to the bubbler gasoutlet fitting 19 to monitor the concentration of the nitrogengas/isopropyl-vapor stream and to detect the presence of any splashingor aerosol effects. The result was that no splashing or aerosol effectswere detected to be present.

What is claimed is: 1) A method for generating a saturated mixture of acarrier gas and a chemical vapor comprising: providing a bubblercontainer having a carrier gas inlet tube and a carrier gas/vaporoutlet, with the bubbler container filled with a chemical liquid, andpassing carrier gas from the gas inlet tube through a plurality of smallgenerator tubes into the chemical liquid exiting therefrom with laminarflow comprising a cylindrical stream for a substantial distance from theoutlet end of the generator tube, and passing output carrier gassaturated with chemical vapor from the chemical fluid through thecarrier gas/vapor outlet, whereby the output carrier gas issubstantially devoid of chemical liquid droplets. 2) The method of claim1 wherein the carrier gas inlet tube passes through the top of thebubbler container and into an enclosed plenum that distributes thecarrier gas to the generator tubes which extend from the bottom of theplenum down into the chemical liquid in the bubbler container. 3) Themethod of claim 1 wherein the dimensions of the generator tubes arechosen such that at the maximum carrier gas flow rate the carrier gasstream exiting the generator tube into the liquid is a high velocityfully developed laminar flow whereby the exiting cylindrical stream ofcarrier gas maintains a small diameter cylindrical shape in the chemicalliquid and as the stream stretches farther away from the outlet end ofthe generator tube, the surface tension at the carrier gas chemicalliquid interface acts to pinch off the cylindrical stream of carrier gasinto a series of small bubbles. 4) Bubbler apparatus for generating asaturated mixture of a carrier gas and a chemical vapor comprising: abubbler container having a carrier gas inlet tube and a carriergas/vapor outlet, with the bubbler container filled with a chemicalliquid, and a plurality of small generator tubes for passing carrier gasfrom the gas inlet tube into the chemical liquid exiting therefrom withlaminar flow comprising a cylindrical stream for a substantial distancefrom the outlet end of the generator tube, whereby the output carriergas passing output carrier gas is saturated with chemical vapor from thechemical fluid through the carrier gas/vapor outlets substantiallydevoid of chemical liquid droplets. 5) The apparatus of claim 4 whereinthe carrier gas inlet tube passes through the top of the bubblercontainer and into an enclosed plenum that distributes the carrier gasto the generator tubes which extend from the bottom of the plenum downinto the chemical liquid in the bubbler container. 6) The apparatus ofclaim 4 wherein the dimensions of the generator tubes are chosen suchthat at the maximum carrier gas flow rate the carrier gas stream exitingthe generator tube into the liquid is a high velocity fully developedlaminar flow whereby the exiting cylindrical stream of carrier gasmaintains a small diameter cylindrical shape in the chemical liquid andas the stream stretches farther away from the outlet end of thegenerator tube, the surface tension at the carrier gas chemical liquidinterface acts to pinch off the cylindrical stream of carrier gas into aseries of small bubbles.